The apparatus and methods of this invention are directed to the rapid creation of precisely aligned layers over large areas of high resolution photoresist images on amorphous substrates, often transparent glass substrates. Significant speed, cost, and alignment tolerance improvements are provided for fabrication of LAED's.
The large size of substrates used to make LAED's (compared to wafer substrates used in IC manufacture) allows one to use two coordinated optical columns, i.e., camera and lens systems, to print the pattern. These two columns, when properly aligned, each print roughly half of an LAED, substantially doubling printing speed over conventional single column steppers since two lenses working simultaneously print twice the area at a time.
When stepping aligners are used to generate arrays of IC patterns, the patterns are later cut into individual chips. The space left during patterning for later cutting, the "scribe line," is often used for local alignment. The stepper disclosed in U.S. Pat. No. 4,040,736, Johannsmier et al., describes such an alignment system. This approach relieves the performance burden placed on stage stepping accuracy, by allowing continuous realignment of the stepping pattern within the array.
The manufacture of LAED's, such as flat panel displays, is substantially different. All of the individual images must abut to tight horizontal tolerances to form an overall integrated, uniform, and precisely interconnected, correlated circuit pattern, with no perceivable joints. Due to the absence of spacing between images, one normally cannot use an alignment mark between images each time the stage is stepped. Rather, one set of alignment marks for the entire array, placed around the outside edge, is used. As a result, an order of magnitude improvement in stage metrology is needed to maintain vertical alignment tolerances. The apparatus of this invention includes special sensor subsystems, appropriate system control software, and machine setup methods to accomplish this improvement.
The behavior of potentially unstable amorphous substrates must also be corrected for in the apparatus, to achieve proper coordination of the optical columns (cameras) when printing later levels of the LAED pattern. After one level of patterning, the partially completed circuit (on the substrate) will be cycled through a thin film process, usually involving a significant temperature cycle. After such a process, the substrate and circuit pattern will likely change in overall size. This "scale" change is measured and compensated for in the system control software, using correcting mechanisms provided in the apparatus.
Historically, the integrated circuit mask-making industry did at one time use multi-barreled repeaters, that is, units with banks of six or nine lenses. For example, Baggaley U.S. Pat. No. 3,563,648 discloses the use of nine parallel optical columns directing images to nine separate masks. The use of multi-barreled repeaters was unsuccessful, however, and the practice ceased about 1974. The problem had been that stage travel was measured in one place and the lens barrels were in another, so inaccuracy resulted. This is because there is a radial component of stage motion (yaw) which causes misalignment. This arises because motion of a stage is not in an exact straight line, but can be off over a short distance by as much as two arc seconds. This could create an error in the projected image of as much as 1.6 um. Since, in making flat panel displays, we are dealing in error factors as low as 0.2 um, an error factor of as much as 1.6 um is unacceptable.
Early steppers such as that made by Baggaley et al. were also fixed focus cameras. The lenses used in these tools were non-telecentric. The magnification of the projected images varied by as much as 3.0 um, due to stage and plate motion up and down under the fixed focus columns. In the apparatus of our invention, asymmetrical telecentric lenses are used, and individual focus control and motion is provided for each optical column to overcome these problems.
Finally, the stepper built by Baggaley, et al., imaged a separate plate for each column. The relationship among the columns (cameras) was therefore not important; the images from the several columns were never integrated into one contiguous image on one plate. The apparatus of our invention must successfully project images in exact spacing, shape, size, and orientation onto one plate, so that an integrated large area electronic device is created from precisely joined images. A method for precisely setting and maintaining the absolute column magnification and spacing must be provided.
It should be noted, also, that Baggaley had no way of repositioning a substrate and no method of adjusting optical columns relative to one another.
Another example of multiple optical columns will be found in Fox U.S. Pat. No. 3,722,996, which discloses the use of dual optical columns. These, however, were not used together; rather, one was for a pattern generating mode and the other for a photorepeater mode. In reality, this was simply two separate machines combined together for economy; and the two units were never used in parallel.
Interferometer systems for controlling stage positioning are described in Baggaley U.S. Pat. No. 3,563,648 and Fox U.S. Pat. No. 3,772,996. In these patents the use of two interferometers on one machine is described, one for each axis.
The use of multiple alignment or reference marks on a wafer, at least one for each chip being etched, will be found in Van Peski et al. U.S. Pat. No. 4,521,114; Meshman et al. U.S. Pat. No. 4,550,374; Suzuki et al. U.S. Pat. No. 4,620,785; Phillips U.S. Pat. No. 4,585,337; and Tanimoto U.S. Pat. No. 4,629,313.